runtime/pprof: slow new goroutine launches in test
runtime/pprof: slow new goroutine launches in test

The goroutine profiler tests include one that launches a steady stream
of goroutines. That creates a scheduler busy loop that can prevent
forward progress in the rest of the program. Slow down the launches a
bit so other goroutines have a chance to run.

Fixes #52916
For #52934

Change-Id: I748557201b94918b1fa4960544a51a48d9cacc6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/406654
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go